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Understanding Operating Systems Fifth Edition

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Title: Understanding Operating Systems Fifth Edition


1
Understanding Operating Systems Fifth Edition
  • Chapter 4Processor Management

2
Learning Objectives
  • The difference between job scheduling and process
    scheduling, and how they relate
  • The advantages and disadvantages of process
    scheduling algorithms that are preemptive versus
    those that are nonpreemptive
  • The goals of process scheduling policies in
    single-core CPUs
  • Up to six different process scheduling algorithms
  • The role of internal interrupts and the tasks
    performed by the interrupt handler

3
Overview
  • Single-user systems (two states)
  • Busy state executing a job
  • Idle state all other times
  • Simple processor management
  • Program (job)
  • Inactive unit
  • File stored on a disk
  • A unit of work submitted by a user
  • Not a process

4
Overview (continued)
  • Process (task)
  • Active entity
  • Requires resources to perform function
  • Processor and special registers
  • Executable program single instance
  • Thread
  • Portion of a process
  • Runs independently
  • Processor
  • Central processing unit (CPU)
  • Performs calculations and executes programs

5
Overview (continued)
  • Multiprogramming environment
  • Processor allocated for a time period
  • Deallocated at appropriate moment delicate task
  • Interrupt
  • Call for help
  • Activates higher-priority program
  • Context Switch
  • Saving job processing information when
    interrupted
  • Single processor
  • May be shared by several jobs (processes)
  • Requires scheduling policy and scheduling
    algorithm

6
About Multi-Core Technologies
  • Processor (core)
  • Located on chip
  • Multi-core CPU (more than one processor)
  • Dual-core, quad-core
  • Single chip may contain multiple cores
  • Multi-core engineering
  • Resolves leakage and heat problems
  • Multiple calculations may occur simultaneously
  • More complex than single core discussed in
    Chapter 6

7
Job Scheduling Versus Process Scheduling
  • Processor Manager
  • Composite of two submanagers
  • Hierarchy between them
  • Job Scheduler higher-level scheduler
  • Job scheduling responsibilities
  • Job initiation based on certain criteria
  • Process Scheduler lower-level scheduler
  • Process scheduling responsibilities
  • Determines execution steps
  • Process scheduling based on certain criteria

8
Job Scheduling Versus Process Scheduling
(continued)
  • Job Scheduler functions
  • Selects incoming job from queue
  • Places in process queue
  • Decides on job initiation criteria
  • Process scheduling algorithm and priority
  • Goal
  • Sequence jobs
  • Efficient system resource utilization
  • Balance I/O interaction and computation
  • Keep most system components busy most of time

9
Process Scheduler
  • Chapter focus
  • Process Scheduler functions
  • Determines job to get CPU resource
  • When and how long
  • Decides interrupt processing
  • Determines queues for job movement during
    execution
  • Recognizes job conclusion
  • Determines job termination
  • Lower-level scheduler in the hierarchy

10
Process Scheduler (continued)
  • Exploits common computer program traits
  • Programs alternate between two cycles
  • CPU and I/O cycles
  • Frequency and CPU cycle duration vary
  • General tendencies exists
  • I/O-bound job
  • Many brief CPU cycles and long I/O cycles
    (printing documents)
  • CPU-bound job
  • Many long CPU cycles and shorter I/O cycles (math
    calculation)

11
Process Scheduler (continued)
  • Poisson distribution curve
  • CPU cycles from I/O-bound and CPU-bound jobs

12
Process Scheduler (continued)
  • Middle-level scheduler third layer
  • Found in highly interactive environments
  • Handles overloading
  • Removes active jobs from memory
  • Reduces degree of multiprogramming
  • Results in faster job completion
  • Single-user environment
  • No distinction between job and process scheduling
  • One job active at a time
  • Receives dedicated system resources for job
    duration

13
Job and Process Status
  • Jobs move through the system
  • Five states
  • HOLD
  • READY
  • WAITING
  • RUNNING
  • FINISHED
  • Called job status or process status

14
Job and Process Status (continued)
15
Job and Process Status (continued)
  • User submits job (batch/interactive)
  • Job accepted
  • Put on HOLD and placed in queue
  • Job state changes from HOLD to READY
  • Indicates job waiting for CPU
  • Job state changes from READY to RUNNING
  • When selected for CPU and processing
  • Job state changes from RUNNING to WAITING
  • Requires unavailable resources
  • Job state changes to FINISHED
  • Job completed (successfully or unsuccessfully)

16
Job and Process Status (continued)
  • Job Scheduler (JS) or Process Scheduler (PS)
    incurs state transition responsibility
  • HOLD to READY
  • JS initiates using predefined policy
  • READY to RUNNING
  • PS initiates using predefined algorithm
  • RUNNING back to READY
  • PS initiates according to predefined time limit
    or other criterion
  • RUNNING to WAITING
  • PS initiates by instruction in job

17
Job and Process Status (continued)
  • Job Scheduler (JS) or Process Scheduler (PS)
    incurs state transition responsibility
    (continued)
  • WAITING to READY
  • PS initiates by signal from I/O device manager
  • Signal indicates I/O request satisfied job
    continues
  • RUNNING to FINISHED
  • PS or JS initiates upon job completion
  • Satisfactorily or with error

18
Process Control Blocks
  • Data structure
  • Contains basic job information
  • What it is
  • Where it is going
  • How much processing completed
  • Where stored
  • How much time spent using resources

19
Process Control Blocks (continued)
  • Process Control Block (PCB) components
  • Process identification
  • Unique
  • Process status
  • Job state (HOLD, READY, RUNNING, WAITING)
  • Process state
  • Process status word register contents, main
    memory info, resources, process priority
  • Accounting
  • Billing and performance measurements
  • CPU time, total time, memory occupancy, I/O
    operations, number of input records read, etc.

20
Process Control Blocks (continued)
21
PCBs and Queuing
  • Job PCB
  • Created when Job Scheduler accepts job
  • Updated as job executes
  • Queues use PCBs to track jobs
  • Contains all necessary job management processing
    data
  • PCBs linked to form queues (jobs not linked)
  • Manage queues using process scheduling policies
    and algorithms

22
PCBs and Queuing (continued)
23
Process Scheduling Policies
  • Multiprogramming environment
  • More jobs than resources at any given time
  • Operating system pre-scheduling task
  • Resolve three system limitations
  • Finite number of resources (disk drives,
    printers, tape drives)
  • Some resources cannot be shared once allocated
    (printers)
  • Some resources require operator intervention
    (tape drives)

24
Process Scheduling Policies (continued)
  • Good process scheduling policy criteria
  • Maximize throughput
  • Run as many jobs as possible in given amount of
    time
  • Minimize response time
  • Quickly turn around interactive requests
  • Minimize turnaround time
  • Move entire job in and out of system quickly
  • Minimize waiting time
  • Move job out of READY queue quickly

25
Process Scheduling Policies (continued)
  • Good process scheduling policy criteria
    (continued)
  • Maximize CPU efficiency
  • Keep CPU busy 100 percent of time
  • Ensure fairness for all jobs
  • Give every job equal CPU and I/O time
  • Final policy criteria decision in designers hands

26
Process Scheduling Policies (continued)
  • Problem
  • Job claims CPU for very long time before I/O
    request issued
  • Builds up READY queue and empties I/O queues
  • Creates unacceptable system imbalance
  • Corrective measure
  • Interrupt
  • Used by Process Scheduler upon predetermined
    expiration of time slice
  • Current job activity suspended
  • Reschedules job into READY queue

27
Process Scheduling Policies (continued)
  • Types of scheduling policies
  • Preemptive
  • Used in time-sharing environments
  • Interrupts job processing
  • Transfers CPU to another job
  • Nonpreemptive
  • Functions without external interrupts
  • Infinite loops interrupted in both cases

28
Process Scheduling Algorithms
  • Base on specific policy
  • Allocate CPU and move job through system
  • Six algorithm types
  • First-come, first-served (FCFS)
  • Shortest job next (SJN)
  • Priority scheduling
  • Shortest remaining time (SRT)
  • Round robin
  • Multiple-level queues
  • Current systems emphasize interactive use and
    response time (use preemptive policies)

29
First-Come, First-Served
  • Nonpreemptive
  • Job handled based on arrival time
  • Earlier job arrives, earlier served
  • Simple algorithm implementation
  • Uses first-in, first-out (FIFO) queue
  • Good for batch systems
  • Unacceptable in interactive systems
  • Unpredictable turnaround time
  • Disadvantages
  • Average turnaround time varies seldom minimized

30
First-Come, First-Served (continued)
31
Shortest Job Next
  • Nonpreemptive
  • Also known as shortest job first (SJF)
  • Job handled based on length of CPU cycle time
  • Easy implementation in batch environment
  • CPU time requirement known in advance
  • Does not work well in interactive systems
  • Optimal algorithm
  • All jobs are available at same time
  • CPU estimates available and accurate

32
Shortest Job Next (continued)
33
Priority Scheduling
  • Nonpreemptive
  • Preferential treatment for important jobs
  • Highest priority programs processed first
  • No interrupts until CPU cycles completed or
    natural wait occurs
  • READY queue may contain two or more jobs with
    equal priority
  • Uses FCFS policy within priority
  • System administrator or Processor Manager use
    different methods of assigning priorities

34
Priority Scheduling (continued)
  • Processor Manager priority assignment methods
  • Memory requirements
  • Jobs requiring large amounts of memory
  • Allocated lower priorities (vice versa)
  • Number and type of peripheral devices
  • Jobs requiring many peripheral devices
  • Allocated lower priorities (vice versa)

35
Priority Scheduling (continued)
  • Processor Manager priority assignment methods
    (continued)
  • Total CPU time
  • Jobs having a long CPU cycle
  • Given lower priorities (vice versa)
  • Amount of time already spent in the system
    (aging)
  • Total time elapsed since job accepted for
    processing
  • Increase priority if job in system unusually long
    time

36
Shortest Remaining Time
  • Preemptive version of SJN
  • Processor allocated to job closest to completion
  • Preemptive if newer job has shorter completion
    time
  • Often used in batch environments
  • Short jobs given priority
  • Cannot implement in interactive system
  • Requires advance CPU time knowledge
  • Involves more overhead than SJN
  • System monitors CPU time for READY queue jobs
  • Performs context switching

37
Shortest Remaining Time (continued)
38
Round Robin
  • Preemptive
  • Used extensively in interactive systems
  • Based on predetermined slice of time
  • Each job assigned time quantum
  • Time quantum size
  • Crucial to system performance
  • Varies from 100 ms to 1-2 seconds
  • CPU equally shared among all active processes
  • Not monopolized by one job

39
Round Robin (continued)
  • Job placed on READY queue (FCFS scheme)
  • Process Scheduler selects first job
  • Sets timer to time quantum
  • Allocates CPU
  • Timer expires
  • If job CPU cycle gt time quantum
  • Job preempted and placed at end of READY queue
  • Information saved in PCB

40
Round Robin (continued)
  • If job CPU cycle lt time quantum
  • Job finished allocated resources released and
    job returned to user
  • Interrupted by I/O request information saved in
    PCB and linked to I/O queue
  • Once I/O request satisfied
  • Job returns to end of READY queue and awaits CPU

41
Round Robin (continued)
42
Round Robin (continued)
  • Efficiency
  • Depends on time quantum size
  • In relation to average CPU cycle
  • Quantum too large (larger than most CPU cycles)
  • Algorithm reduces to FCFS scheme
  • Quantum too small
  • Context switching occurs
  • Job execution slows down
  • Overhead dramatically increased

43
Round Robin (continued)
44
Round Robin (continued)
  • Best quantum time size
  • Depends on system
  • Interactive response time key factor
  • Batch turnaround time key factor
  • General rules of thumb
  • Long enough for 80 of CPU cycles to complete
  • At least 100 times longer than context switch
    time requirement

45
Multiple-Level Queues
  • Works in conjunction with several other schemes
  • Works well in systems with jobs grouped by common
    characteristic
  • Priority-based
  • Different queues for each priority level
  • CPU-bound jobs in one queue and I/O-bound jobs in
    another queue
  • Hybrid environment
  • Batch jobs in background queue
  • Interactive jobs in foreground queue
  • Scheduling policy based on predetermined scheme
  • Four primary methods

46
Case 1 No Movement Between Queues
  • Simple
  • Rewards high-priority jobs
  • Processor allocated using FCFS
  • Processor allocated to lower-priority jobs
  • Only when high-priority queues empty
  • Good environment
  • Few high-priority jobs
  • Spend more time with low-priority jobs

47
Case 2 Movement Between Queues
  • Processor adjusts priorities assigned to each job
  • High-priority jobs
  • Initial priority favorable
  • Treated like all other jobs afterwards
  • Quantum interrupt
  • Job preempted
  • Moved to next lower queue
  • May have priority increased
  • Good environment
  • Jobs handled by cycle characteristics (CPU or
    I/O)
  • Interactive systems

48
Case 3 Variable Time Quantum Per Queue
  • Case 2 variation movement between queues
  • Each queue given time quantum size
  • Size twice as long as previous queue
  • Fast turnaround for CPU-bound jobs
  • CPU-bound jobs execute longer and given longer
    time periods
  • Improves chance of finishing faster

49
Case 4 Aging
  • Ensures lower-level queue jobs eventually
    complete execution
  • System keeps track of job wait time
  • If too old
  • System moves job to next highest queue
  • Continues until old job reaches top queue
  • May drastically move old job to highest queue
  • Advantage
  • Guards against indefinite unwieldy job
    postponement
  • Major problem discussed further in Chapter 5

50
A Word About Interrupts
  • Interrupt Types
  • Page interrupt (memory manager)
  • Accommodate job requests
  • Time quantum expiration interrupt
  • I/O interrupt
  • Result from READ or WRITE command issuance
  • Internal interrupt
  • Synchronous
  • Result from arithmetic operation or job
    instruction
  • Illegal arithmetic operation interrupt
  • Dividing by zero bad floating-point operation

51
A Word About Interrupts (continued)
  • Interrupt Types (continued)
  • Illegal job instruction interrupt
  • Protected storage access attempt
  • Interrupt handler
  • Control program
  • Handles interruption event sequence

52
A Word About Interrupts (continued)
  • Nonrecoverable error detected by operating system
  • Interrupt handler sequence
  • Interrupt type described and stored
  • Interrupted process state saved
  • Interrupt processed
  • Processor resumes normal operation

53
Summary
  • Processor Manager allocates CPU among all users
  • Job Scheduler
  • Assigns job to READY queue
  • Based on characteristics
  • Process Scheduler
  • Instant-by-instant allocation of CPU
  • Scheduling algorithm is unique
  • Characteristics, objectives, and applications
  • System designer selects best policy and algorithm
  • After careful strengths and weaknesses evaluation

54
Summary (continued)
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